1. Field of the Invention
The invention relates to two color imagers, and more particularly to an imager for simultaneously obtaining two images of differing color bands of the same object field using the same two dimensional photodetector array.
The invention, which utilizes a photodetector array operating a staring mode, has application to those portions of the visible and invisible spectrum to which a photodetector array responds, including the infrared (IR) spectrum.
2. Prior Art
In many optical systems applications there is a need to perform two-color imaging. Two color imaging is of substantial importance in applications such as aerial mapping, where both visible and invisible light can play an important part in gathering geological data, determining surface temperatures, vegetation, etc., as well as in infrared systems where it is desirable to recognize an object by a "signature" composed of two color bands in the IR spectrum.
A duplicated system for two color operation in any portion of the spectrum accessible to photodetector arrays is not commercially attractive due to the high cost of an additional staring array, and of additional read out electronics. For IR operation, where an additional dewar/cooler assembly may be needed, the use of a multiple array approach is even more costly. The cost and system complications makes the multiple array approach for two color operation in any portion of the spectrum unattractive.
One known approach for obtaining two color operation from a single area array is that of using color "checkerboard filters", placed in close proximity to the array on the focal plane of an imager. In this approach, pixel level filters may be employed. This approach suffers from several problems. First, the checkerboard filters are expensive to fabricate and due to practical limits on filter design, the spatial transition from one filter element to the next requires an appreciable fraction of the array pixel dimension, even with large pixels (a pitch of 3-4 mils). Consequently, the checkerboard filter reduces the optical fill factor of the array. Mounting the checkerboard filter in close proximity to the array is difficult and adds to the cost of the focal plane due to both added complexity in focal plane array (FPA) assembly and added criticality in the alignment between filter and photodetector array. Pixel level checkerboard filters must be mounted with an accuracy of a fraction of a pixel (often less than 1 mil) with respect to the active surface of the array.
There is, therefore, a need for a simplified means for simultaneously obtaining two-color images using a single area array and requiring neither duplicated readout electronics nor--in the case of operation in the IR spectrum of a duplicated cryogenic environment.